Technological development and increased demand for mobile equipment have led to a rapid increase in the demand for secondary batteries as energy sources. Recently, use of secondary batteries is realized as power sources of electric vehicles (EVs), hybrid electric vehicles (HEVs) and the like. Accordingly, a great research is focused on secondary batteries satisfying various requirements and, in particular, use of lithium secondary batteries with high energy density, high discharge voltage and superior output stability is increasing.
In particular, lithium secondary batteries used for electric vehicles have high energy density, exhibit great power within a short time and should be used under severe conditions for 10 years or longer, thus requiring considerably superior stability and long lifespan, as compared to conventional small lithium secondary batteries.
In addition, recently, a great deal of research is focused on use of lithium secondary batteries for power storage devices in which unusable power is converted into physical or chemical energy, stored and used as electric energy, as necessary.
Lithium secondary batteries used for large-capacity power storage devices should have high energy density and efficiency, and long lifespan, and in particular, should secure safety and reliability, since combustion or explosion during malfunction of systems may cause major accidents.
In this regard, conventional lithium secondary batteries generally utilize a lithium cobalt composite oxide having a layered structure for a cathode and a graphite-based material for an anode. However, such lithium cobalt composite oxide is disadvantageous in that cobalt used as a main element is extremely expensive and a layered structure is unsuitable for electric vehicles or large-capacity power storage devices in terms of safety in that it undergoes variation in volume due to repeated intercalation and deintercalation of Li cations and is deformed when half or more of the Li cations are deintercalated.
In addition, lithium cobalt composite oxide has a spinel structure having a 3-dimensional interstitial space and does not undergo variation in volume due to intercalation and deintercalation of Li cations, but manganese is eluted into an electrolyte solution due to the effect of the electrolyte solution during charge and discharge at high temperature and high current, thus disadvantageously deteriorating batter characteristics, and having a limitation of increase in capacity per unit weight due to small capacity per unit weight, as compared to lithium cobalt composite oxides or lithium nickel composite oxides. Accordingly, a great deal of research is focused on a novel cathode active material having other crystal structure.
For example, there is hollandite such as α-MnO2, as a tunnel structure having a 1-dimensional interstitial space. A great deal of research has focused on this material since stable intercalation and deintercalation of Li cations are thought to be possible in the tunnel.
However, metals having a large ion size such as barium (Ba) and cesium (Cs) stably constitute a hollandite structure, while a structure that does not contain these metals is relatively unstable and exhibits poor lifespan.
Accordingly, attempts to solve these problems by doping with transition metals such as Co have been made. In accordance with recent developments in nanotechnology, 1-dimensional path of Li cations can be further decreased, and such a tunnel structure is actively researched.
However, no satisfactory material has been developed to date.